Insulating oil composition



I Patented 7, 1941 UNITED stares PATENT OFFICE 4 2,257,871

msuna'rmo on. comrosmoN Charles E.

t No Drawing.

Trantman, East McKeesport, l'aq assignor to Gulf Research pany,Pittsburgh, Pa... a

& Development Comcorporation of Dela- Application November 9, 1940,Serial No. 365,128

I iclaiina. ((1252-64) This invention relates to improved insulating oilcompositions and it comprises improved trans- I former oils and otherinsulating oil compositions containing in admixture therewith relativelysmall amounts of trithiophenyl phosphate, the

- amount of said improvement agent being sufflcient to stabilize thevpetroleum oil against I oxidation; particularly against sludgeformation,

' to such undesirableoxidational changes; all asmore fully hereinafterset forth and as claimed.

In the present invention, transformer oils and like insulating oils areimproved by incorporating in the oil or oil composition, a small amountof trithiophenyl phosphate, the amount added being suiilcient tostabilize the oil against oxidation, as stated ante. And the amounts ofsuch additions necessary to effectively stabilize the usual commercialoils do not exceed 1 per cent by weight of the oil.- For instance, byadding 0.1 to 0.5 per cent of trithiophenyl phosphate to transformeroils, I have obtained oil compositions which are stable againstoxidation. This is more fully shown post. And as there shown, my new andimproved oils are superior to the original .oils and have a much higherresistance to sludging and other improved properties.

The improvement agent here used, namely, trithiophenyl phosphate is awell known chemical compound. It is readily soluble in petroleum oils inthe amounts here employed and it does not. deleteriously affecteitherthe physical or elec-' trical properties of transformer oils whenincorporated thereinin the amounts necessary for the present purposes.

Further, trithiophenyl phosphate is a substantially neutral compound,having the following structural formula:

Thus. it is also a thio-ether compound, each of development of acidityand other undesirable the sulfur atoms being Joined to a phenyl radical,as shown in the above formula. In fact this compound may be obtained byreacting phenyl mercaptan or its alkali metal salts withphosphorus'oxychloride (OPCls) and recovering the trithiophenylphosphate so obtained. While this compound is soluble in petroleum oils,it is insoluble inwater. And when dissolved in transformer oils in smallamounts, it improves their properties as fully shown post. And as thereshown, my new and improved compounded are superior to the original oilsand my compounded oil compositions, because of their superior resistanceto oxidation and other improved properties, possess exceptional utilityas transformer oils.

In many types of electric transformers the core and windings areimmersed in a suitable petroleum oil, which serves-as a dielectric andinsulating oil and for carrying heat away from the core and windings.The requirements of a] transformer oil include good dielectricproperties,

low viscosity to facilitate heat transfer by convection, low pour pointto avoid solidification of the oil by cold, high flash and fire pointsto minimize danger of fire and explosion and low volatility to reducevapor losses. In addition,

the oil must be resistant to oxidation and slud -ing-, and must be ofsuch character as to resist emulsiflcation with water.

Transformer oils ordinarily are distillate oils refined in such manneras to secure the specified characteristics to as great an extent aspossible. Achievement of satisfactory oxidation resistance is difficult,for while oxidational changes are slow, transformer oils are ordinarilyexpected to remain in service for long periods. Oxidational changes mayresult in the formation of insoluble matter or sludge. The sludge tendsto render the oil more misciblewith water, with production of emulsionsin the presence of condensed water and a consequent degradation of thedielectric properties of the oil body. deposits on the windings and corewith reduction of the efiiciency .of heat transfer. Oxidation oftransformer oils tends to be promoted, by contact with metals,especially copper and with the usual varnishes and insulating materials.

Various materials have been proposed for incorporation into transformeroils with the object of inhibiting oxidation, and some of these havecome into use. The usual anti-oxidants may be divided into twoprincipalclasses: (1) those which are associated with a lengthening ofthe inductionperlodthat is, the timewhich-elapses The' sludge alsoinsulating oils are readily obtained.

before appearance of sludge in the oil subject to oxidation, and (2)those which reduce the rate at which sludge is formed after it oncebegins to form. In evaluating oils containing anti-oxidants, the mostuseful criterion is a value which takes into'acoount both of thesecharacteristics viz., the time required, under the test conditions, toform a definite amount of sludge, say, 0.1 per cent by weight of theoil.

I have discovered that trithiophenyl phosphate brings about alengthening of the induction period and a lessening of the rate ofsludge formation thereafter when incorporated in transformer oils evenin minimal amounts. As a result of these two inherent properties oftrithiophenyl phosphate, compounded transformer oils containing thisagent have an improved over-all efflciency under service conditions. Andsuch advantageous improvements can be obtained without deleteriouslyaffecting the electrical and physical properties of the transformer oil.For instance, the resistance of the oil to absorptiomof moisture and toemulsiflcation are not lowered; trithiophenyl phosphate being insolublein water as stated ante. If anything, those properties of thetransformer oil are also improved by the addition of trithiophenylphosphate. 0n the other hand, trithiophenyl phosphate is readily solublein transformer oils in the amounts necessary to obtain the desiredimprovements in resistance to oxidation. In other words, trithiophenylphosphate is fully miscible with such oils initially and compatible withthem under service conditions.

That is, I have found that triphenyl phosphate is a useful improvementagent for insulating oils and has several advantages as an improvementagent for such oils, particularly transformer oils.

In preparing my improved transformer oils, various petroleum oils may beemployed and the amounts of trithiophenyl phosphate added to the oil maybe varied to give the improved stability and other improvements desired.In general, the

amounts of trithiophenyl phosphate employed in accordance with myinvention are not in excess of 1 per cent by weight on the oil to beinhibited.

I have foundthat 0.1 per cent of trithiophenyl phosphate is eflectivebut this may be varied in accordance with the results desired, althoughit should not ordinarily be necessary to employ more than 0.5 per centof trithiophenyl phosphate with the usual transformer oils. Of course.the exact amounts required to secure the desired improvements vary withthe particular oil'employed.

-In the pract ce of the present invention, the oil bases to which thisagent is added are the usual transformer oils ordinarily comprisinghighly refined petroleum distillates having the physical characteristicswithin the ordinary specifications .for such oils. The acid-refined andsolvent-refined oils may be employed. In some instances it is desirableto employ oils which have been subjected both to acid-refining andsolventrefining methods.

By incorporating the said small mounts of trithiophenyl phosphate insuch oils.- improved The imformation test for mineral transformer 011.de-

scribed by I". M. Clark and E. A. Snyder in their paper entitled Testingfor Sludge Formations in Mineral Transformer 0 presented at thethirty-ninth annular meeting for the American 50- ciety for TestingMaterials. I

In the tests reported in said table, samples of oil and of thecompounded oils were maintained in tubes containing copper wire at aconstant temperature of 120 C. The tubes were opened at the top to anenclosed, air space and dry air was passed at a controlled rate of 1 to2' cubic feet per hour through the enclosed space above the sampletubes. In each tube was introduced 22 grams of oil or oil composition tobe tested,

a number of tubes being employed. At prescribed intervals the tubes weresuccessively removed and the amount of sludge determined. The results ofthese tests are reported in terms of the induction period, the per centof-sludge formed per day after the induction period and the total numberof days required to form 0.1 per cent of sludge.

The oil employed in all of the tests set forth herein below was atypical petroleum transformer oil commonly used for the purposeindicated. The results obtained in testing the oil alone and in testingthe oil together with varying amounts of trithiophenyl phosphate are asfollows:

From the data given in the above table, it is evident that myimprovedrtransformer oil containing trithiophenyl phosphate are superiorto the original transformer oil. For instance, by comparing the resultsobtained with the original oil No. 1 against those obtained with thetransformer oils 2, 3 and 4 (made in accordance with the presentinvention) it will be seen that my improved'transformer oils are muchmore stable against sludging than the original oil.

For one thing, my improved oils have an induction period greater thanthat of the original oil.

proved oil compositions so obtained are resistant 4 to sludging andother undesirable oxidational changes under service conditions.

The data given in the table post will serve to illustrate the practiceof my invention and the utility thereof. The test data given in thetable were obtained in accordance with the sludge For example, myimproved transformer oil No. 2

has an induction period three times that of the original oil. That is,even as little as 0.1 per cent of trithiophenyl phosphate. inhibitsoxidation. And by adding larger amounts, further improvements can beobtained. This is shown in the case of oils Nos. 3 and 4 which contain0.3 to 0.5

per cent, respectively, and show induction period six and twelve timesgreater than that of the original oil. F v

Another and even .more important improvement obtainable by means oftrithiophenyl phosphate is shownby the data on the rate of sludgeformation given in the table. As shown by that data, the incorporationof small amounts of trithiophenyl phosphate in the: oil markedlydecreases the rate of sludge formation after the induction period. Forexample, by comparing oil aasam N0. his. the original oil (No.11), itwill be seen that as little as 0.1 per cent of trithiophenyl phosis a 35and 40 per cent reduction in the rate of sludge formation, respectively.In other words, my improved transformer oils containing small.

amounts of trithiophenyl phosphate dissolved therein form much lesssludge per day' than the original oil.'

Thus my improved transformer oils have a longer induction period and .amuch lower rate of sludge formation after the induction period. than theoriginal oil. Consequently they have a much greater resistance tosludgln and this over-all superiority is evident from the data on thetotal time to form 0.1 per cent sludge given in the table. For example,transformer oils 2, 3 and 4 are from 40 per cent to 180 per cent moreresistant to sludglng than theoriginal oil as determined by saidaccelerated test and expressed in terms of thetotal time toform 0.1 percent sludge. I

From the facts and data given ante. it is evident that my improvedtransformer oils containing trithiophenyl phosphateare much superior tothe original transformer oil as to resistance to sludging and otheroxidational changes.

While my invention has been described herein and illustrated ante withparticular reference to transformer oil compositions, trithiophenylphosphate is equally effective in stabilizing other electricalinsulating oils against oxidation and deterioration. And in addition toimproving transformer oils, I can also improve other insulatingphosphate in the oils by the present invention.

In improving insulating oils generally, I like-. wise incorporate smallamountaof trithiophenyl oil. The amount added to the oil may be varieddesired. By the present invention, various types of insulating oils.such as cable oils, switch oils, circuit breaker oils. etc., as well astransformer oils, may beimproved by incorporating sumcient trithlophenylphosphate in the oil to render it highly resistant to sludging and otherdeleterious oxidational changes under service conditions, withoutdeleteriously affecting the electrical and physical properties of theoil necessary for the various uses thereof as an insulating oil;

What I claim is:

1. In the operation of electrical transformers containing a petroleumoil as a dielectric. and cooling medium, the improvement which comprisesntaining a small amount of trithiophenyl p osphate in said oil tosubstantially inhibit oxidational changes which would otherwise takeplace in said all during normal conditions of such use. I

2. The process of claim 1 wherein theamount of trithiophenylphosphate-maintained in said oil is from 0.1 to 0.5per cent of the oil.

3. In the operation of electrical transformers containing a petroleumoil as a dielectric and cooling medium, the improvement which comprisesinsulating and cooling the windings and core of 'the transformer with apetroleum oil containinga small amount of .trithiophenyl phos-'according to the particular insulating oil employed to obtain theimprovement

